Why does the passenger side window on my car state 'objects in mirror are closer than they appear?

Asked by: Suzanna


I think if you are in the U.S., the actual correct answer would be 'so that they may not be sued for damages later'. Fun aside, there is a simple physical explanation for that warning on the passenger side window.

First, let us note that as human beings, one of the clues for distance perception is the apparent size of the object we are looking at, compared to the 'real' size. For example, if you see a person that looks about the same size as your thumb with the arm stretched out, it's pretty certain the person is quite far away, and not a tiny person close-up. Although there are other clues for depth perception, they are mostly useless when observing objects through a small mirror.

If you inspect both side mirrors on your car, you will find out that the driver side mirror is planar, while the passenger side mirror is (convex) spherical. The driver side mirror, being planar, does not alter the size of the objects, they look about the same size in the mirror as they would if you turned your head around and looked at them directly. (Well, almost. The difference is about twice the distance from your eye to the mirror.)

For the passenger side mirror, a planar mirror would not do, since since it is further away from your eye, it would be very limited in the range of vision it provides. So, we need either a huge mirror (not a very good or elegant alternative) or a mirror that can 'compress' images to that smaller size. The latter has been chosen, and the convex mirror is the tool that does the job of 'compression'. Just like the back of a spoon, anything in a convex mirror looks smaller than its actual size.

I suppose by now, you have figured out what the point is: Objects do look smaller in the passenger side mirror than they would if you looked directly at them. So, you _may_ perceive them as being further behind you than they actually are. This may cause you to try to pull off a stunt which you would not have attempted if you knew they were right behind you. So, the warning is there to tell you of this possibility (and again, to avoid getting sued!).

As a side point, looking at the surroundings in the mirror (road lines etc.) helps better perceive the distance of the cars behind you. However, don't keep staring at the mirror too long, or you might fail to see what's in front of you! (This warning is here so that _I_ won't get sued! :-))
Answered by: Yasar Safkan, Ph.D., Software Engineer, Noktalar A.S., Istanbul, Turkey